Conventionally, in almost all liquid crystal display devices, types of which include an electronic paper display device and a liquid crystal display device, a glass substrate has been utilized as the upper and lower substrates, and, in order to achieve lightness of weight of the display device, the upper and lower glass substrates have been made thinner. Currently, although the thickness of the substrate is maximally thinned, satisfactory lightness of weight is difficult to obtain, and thus thorough research into materials to be used for the substrate is being conducted. In this regard, in lieu of the glass substrate, the use of a plastic substrate which is lighter than glass is being proposed.
However, the plastic substrate may suffer from thermal, chemical and mechanical damage in the course of forming an array pattern, forming a color filter, etc., undesirably deteriorating the image quality properties of the liquid crystal display device.
A typical example of a liquid crystal display device including a plastic substrate is schematically illustrated in FIG. 1A. Specifically, the liquid crystal display device may include upper and lower substrates 1 made of plastic, upper and lower electrodes 11 formed on the upper and lower substrates to apply driving voltage to the device and which are provided as transparent electrodes, barriers 140 having a two-layer structure and enabling the upper and lower substrates to be disposed at a predetermined distance, and a layer 150 of electrically charged particles or liquid crystals injected into cells defined by the barriers. Also, a polarizer may be formed on the outer surface of a plastic insulating substrate used as the upper substrate, and a reflective plate or a transmissive substrate may be formed on the outer surface of the lower substrate. Furthermore, the upper plastic insulating substrate may include a black matrix for defining unit pixels, and a color filter may be provided in the space enclosed by the black matrix. On the other hand, the lower substrate may include a switching element and a pixel electrode per unit pixel.
As another example of the flat panel display device, an organic electroluminescent (EL) device illustrates its cross-section in FIGS. 1B and 1C, which includes a substrate 1 and a transparent electrode 110 or a reflective electrode 113 formed on the substrate 1.
Although the thickness and weight of the device are reduced thanks to the use of the plastic substrate, it is still very hard to suppress the deterioration of the image quality properties of a final product and integrate various functionalities so as to reduce the thickness and weight of the device.
Meanwhile, a reflective liquid crystal display device reflects external light to thus attain illumination, and thus includes a reflector for diffusing and reflecting external light. As such, the reflector is provided as an outward reflecting type in which a reflector is formed on the surface of a substrate opposite the surface having a liquid crystal layer, or as an inward reflecting type in which a reflector is formed on the surface of a substrate having a liquid crystal layer.
In accordance with the outward reflecting type, because the reflector is formed on the surface of the substrate opposite the surface having the liquid crystal layer, light incident from the outside is diffused and reflected by the reflector, and the reflected light is sequentially transmitted through the substrate and the liquid crystal layer and is then emitted at a display screen. As such, the reflected light may cause parallax attributable to the influence of the thickness of the substrate, resulting in a defocused image such as a double image or color-blending.
On the other hand, in accordance with the inward reflecting type, because the reflector is formed on the surface of the substrate having the liquid crystal layer, light incident from the outside is diffused and reflected by the reflector, and the reflected light is transmitted through the liquid crystal layer without passing through the substrate, and is then emitted at a display screen. In this case, because the reflector and the liquid crystal layer are disposed adjacent to each other and the reflected light is not affected by the thickness of the substrate, parallax does not occur, and thereby a defocused image is eliminated.
In the case where the substrate is a plastic substrate, namely, a film, the thickness thereof may become thinner than that of glass, and thus parallax may be reduced to that extent. However, in the case where a color filter is provided, one pixel is divided into three color dots each having one-third the size of the pixel. In this case, the reflected light may be transmitted through untargeted dots more easily than in a monochromic liquid crystal display device, and thus, the influence of the film thickness is not negligible.
Even in the case where the film is used as the substrate, particularly for a colored liquid crystal display device, the inward reflecting type should be adopted.
However, the liquid crystal display device of the inward reflecting type has a complicated configuration, thus making it difficult to manufacture. In particular, in the case where a film is used as the substrate, the film may easily expand and contract under the influence of heat or humidity, and so materials or process conditions thereof are restricted. Hence, methods of manufacturing the liquid crystal display device of the inward reflecting type using a film as the substrate are being sought, which are high yield and reduce the manufacturing cost depending on the design requirements.
In addition, in the fabrication of a reflective or transflective liquid crystal display device, a method of forming the reflector includes roughening the surface of a resin such as a resist layer through photolithography using a photomask, and forming a metal layer facilitating the reflection of light on the rough surface.
Other examples of the method of roughening the surface of the resin include a method of forming a rough surface using a resin having small particles dispersed therein (Japanese Unexamined Patent Publication No. Hei. 4-267220), a method of forming a rough surface through phase separation of two different types of resin upon curing (Japanese Unexamined Patent Publication No. Hei. 12-193807), and a method of forming a rough surface by controlling internal stress upon curing of a heat-photocurable resin material (Japanese Unexamined Patent Publication No. Hei. 12-171792).
However, the case where an electrode substrate of the liquid crystal display device of the inward reflecting type using a film as the substrate is manufactured by directly forming the reflector on the film has the following problems. Specifically, because the reflector needs a function of diffusing and reflecting external light, it has a rough surface. This roughness is obtained by roughening the surface of the liquid crystal layer and thus may adversely affect driving of the liquid crystals. So, sophisticated planarization technology is accordingly required, which may increase the manufacturing cost and reduce the yield. Also, the reflector formed of a metal layer may be damaged in the course of subsequent chemical treatment performed in forming the transparent electrode or the like. Furthermore, the film used as the substrate tends to expand or contract under the influence of heat or humidity, restricting the materials and manufacturing conditions for the reflector and the transparent electrode, and thus making it difficult to manufacture the liquid crystal display device depending on the design requirements. Moreover, in the manufacture of an electrode substrate for a transflective liquid crystal display device which is designed to use both a backlight and reflected external light as light sources, by means of directly forming a reflector or the like directly on a film, the problems as mentioned above may become more serious.
In the case where the reflector is formed as the inward reflecting type, a reflector composed of a metal layer is first formed on the entire upper surface of the plastic film. Then, a portion of the metal layer, corresponding to a pixel portion of a transparent electrode to be formed in a subsequent process, needs to be removed using high precision in order to allow the backlight to be transmitted therethrough. As such, the portion to be removed has an area smaller than the pixel portion of the transparent electrode.
Thereafter, ITO serving as the transparent electrode is formed in such a manner that the pixel portion of the transparent electrode overlaps the area in which the reflector composed of the metal layer is removed. A resist layer is formed on the ITO while aligning the resist layer and the ITO with good precision, and is then exposed and developed to thus remove the ITO, thus forming a pattern of the transparent electrode. This process requires a high degree of alignment precision.
However, a film made of plastic expands easily just by water washing, for example, and, conversely, the film contracts when being dried. In addition, such expansion or contraction is not immediately stabilized, and requires a long period of time to stabilize. For example, once the film is contracted, the dimension of the pattern formed on the film shows expansion for a long period of time, making it difficult to obtain reproducibility of the above-mentioned alignment.
Therefore, it is difficult to manufacture the transflective liquid crystal display device by forming the reflector or the transparent electrode directly on the film.
Also, the conventional method of manufacturing the reflector of the reflective (transflective) liquid crystal display device has the following problems.
In the method of creating roughness on the surface of the resin such as the resist layer using photolithography, simple repetition of the planar patterns on the reflector may cause the reflector to function as a diffraction grating. Accordingly, when the liquid crystal display screen is viewed, defects such as iridescence or so-called moire fringes may occur due to subtle errors in positioning relative to other repetitive patterns of wiring, black matrix or the like. For this reason, simple duplicative patterns such as pixel patterns cannot be used in designing the photomask for forming the rough patterns, and extremely vexatious and complicated designing is required. Thus, it is not easy to form reflectors having proper diffusing capacity by typical photolithography.
Also, the method disclosed in Japanese Unexamined Patent Publication No. Hei. 4-267220 disperses particles of a different resin material in the resin, and the method disclosed in Japanese Unexamined Patent Publication No. Hei. 12-193807 utilizes the phase separation of two different types of resin upon curing. Also, the method disclosed in Japanese Unexamined Patent Publication No. Hei. 12-171792 forms roughness by curing a portion of the surface of the resin and performing exposure or burning while leaving its interior uncured. In the case where the reflector is directly formed on a plastic film 50˜200 μm thick using the aforementioned method, the warping of the plastic film may easily occur due to stress upon curing of the resin.
In addition, the reflector manufactured in accordance with any of the foregoing manufacturing methods has a composition of materials that are virtually different in refractive indices on opposing sides of a boundary such as an interface of the resin and the particles or an interface of separated phases. When the rough surface of the reflector is formed on the surface of the electrode substrate opposite the surface having the liquid crystal layer, incident light polarized by a polarizer is transmitted through the interface of the materials of the reflector having the different refractive indices via the liquid crystal layer, and then reflected by the metal layer.
In this case, the polarized incident light and the light dispersed by the metal layer may show depolarization due to refraction at the interface of the materials of the different refractive indices, whereby the degree of polarization of the light is reduced. Accordingly, in the case where the reflector disposed as above is applied to the liquid crystal display device, it is likely that the contrast ratio of the liquid crystal display screen will be reduced.